Ligand-directed top-down synthesis of trivacant lacunary polyoxomolybdates from plenary Keggin-type [α-XMo₁₂O₄₀]³⁻ (X = P, As, V) in organic media

Abstract

Lacunary polyoxometalates (POMs), featuring highly reactive vacant sites, serve as valuable building blocks and precursors for the rational design of functional materials with widespread applications in catalysis, analytical chemistry, energy conversion and storage, medicine, and optical materials. While diverse Keggin-type polyoxotungstates, including both plenary and lacunary species, have been synthesized through dehydration condensation reactions and equilibrium displacement in aqueous solvents, the isolation and use of lacunary polyoxomolybdates remain challenging. To address this, the current study proposes a “ligand-directed top-down synthetic approach” for producing lacunary polyoxomolybdates from plenary Keggin-type species in organic solvents. By reacting plenary Keggin-type polyoxomolybdates [α-XMo₁₂O₄₀]³⁻ (X = P, As, V) with 4-methoxypyridine (pyOMe) in acetonitrile, we successfully synthesized the corresponding trivacant lacunary polyoxomolybdates ([XMo₉O₃₁(pyOMe)₃]³⁻), where the vacant sites are stabilized by three pyOMe ligands. Remarkably, this approach enables the synthesis of a lacunary vanadomolybdate, a species previously unattainable through equilibrium control in aqueous systems. Furthermore, the reversible coordination of pyOMe ligands to molybdenum atoms at the vacant sites makes these lacunary polyoxomolybdates highly versatile precursors for assembling POM–organic hybrids. Overall, this study introduces an innovative synthetic methodology for POMs, demonstrating notable potential for advancing the development of functional materials.

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Introduction

Polyoxometalates (POMs), a class of precisely defined anionic metal oxide clusters (e.g., W⁶⁺, Mo⁶⁺, and V⁵⁺), have garnered considerable attention owing to their structural diversity and remarkable physicochemical properties.^1,2 These clusters are highly versatile, with widespread applications in catalysis, analytical chemistry, energy conversion and storage, medicine, and optical materials. This versatility stems from their tunable properties, including redox potentials, acidity, and stability, which can be finely adjusted by modifying their structures, constituent elements, and oxidation states. Among the various types of POMs, Keggin-type POMs [XM₁₂O₄₀]ⁿ⁻ stand out for their ability to exhibit diverse properties depending on their central heteroatoms (X; e.g., P⁵⁺, As⁵⁺, Si⁴⁺, Ge⁴⁺) and polyatoms (M; e.g., W⁶⁺, Mo⁶⁺). These compounds are typically synthesized through dehydration condensation and equilibrium displacement in aqueous solvents, with careful control over pH conditions, metal ion concentrations, and mixing ratios. To date, extensive research has been conducted on the equilibrium and speciation profiles of polyoxotungstates in aqueous solvents,³ enabling the isolation of fully occupied plenary [XW₁₂O₄₀]ⁿ⁻ species, along with various lacunary species including monovacant [XW₁₁O₃₉]ⁿ⁻, divacant [XW₁₀O₃₆]ⁿ⁻, and trivacant [XW₉O₃₄]ⁿ⁻ polyoxotungstates (e.g., X = P⁵⁺, As⁵⁺, Si⁴⁺, Ge⁴⁺) (Fig. 1a).⁴ These lacunary polyoxotungstates, with reactive vacant sites, serve as valuable building blocks and precursors for designing functional materials.⁴ For instance, our group synthesized various advanced materials, such as multinuclear metal-oxo nanoclusters,⁵ metal nanoclusters,⁶ and organic–POM hybrids, using lacunary POMs in organic solvents.⁷

Fig. 1 (a) Typical synthesis of lacunary polyoxomolybdates via equilibrium control in aqueous solvents. (b) Proposed approach: ligand-directed top-down synthesis of trivacant lacunary polyoxomolybdates (I_X, TBA₃[XMo₉O₃₁(pyOMe)₃], X = P, As, V) from plenary species [XMo₁₂O₄₀]³⁻ in organic solvents and their application in the synthesis of polyoxomolybdate–organic hybrids (II_X).

Polyoxomolybdates, another prominent class of POMs, are characterized by electrochemical, photochemical, and physicochemical properties that substantially differ from those of their tungsten-based counterparts.⁸ Similar to polyoxotungstates, the speciation profiles of phosphomolybdates (P/Mo),^9a,b arsenomolybdates (As/Mo),^9c,d and vanadomolybdates (V/Mo)^9e in aqueous solvents have been extensively researched. Recently, computational studies have further explored the speciation profiles of phosphomolybdates^10a and arsenomolybdates.^10b However, achieving structural control of polyoxomolybdates through equilibrium displacement in aqueous solvents remains challenging, partly because most lacunary species exist as minor species in equilibrium mixtures (Fig. 1a). Although multivacant polyoxomolybdates are promising as building blocks for functional materials owing to the high reactivity of their vacant sites and their distinctive redox properties, the synthesis of these compounds using aqueous synthetic mixtures and their subsequent characterization remain challenging. To date, the trivacant lacunary Keggin-type phosphomolybdate [PMo₉O₃₄]⁹⁻ is the only multivacant polyoxomolybdate that has been isolated from aqueous mixtures.¹¹ The use of multivacant lacunary polyoxomolybdates is further limited by their inherent structural instability. For instance, [PMo₉O₃₄]⁹⁻ is prone to structural transformations or decomposition during reactions with metal ions or organic ligands in both aqueous and organic solvents.¹² To address these challenges, our group recently developed a method to stabilize [PMo₉O₃₄]⁹⁻ by coordinating pyridine ligands to Mo atoms at the vacant sites in organic solvents.¹³ This reversible coordination afforded a pyridine-stabilized lacunary phosphomolybdate ([PMo₉O₃₁(py)₃]³⁻; py = pyridine), serving as a versatile precursor for incorporating metal ions¹⁴ and constructing POM–organic hybrids.^13,15 Using this method, we also achieved a ligand-directed structural transformation of [PMo₉O₃₄]⁹⁻ into a novel divacant lacunary polyoxomolybdate, [PMo₁₀O₃₄(py)₂]³⁻,^15a which has not been previously observed in aqueous POM chemistry.^3,9,10

Building on these insights, we hypothesized that equilibrium control using organic protecting ligands in organic solvents could mitigate undesired hydrolysis, condensation, and structural decomposition. This study aimed to develop a reproducible method for synthesizing and stabilizing new lacunary polyoxomolybdates while enabling their use in functional material assembly. Accordingly, in this study, we propose a “top-down synthetic approach” for preparing lacunary Keggin-type polyoxomolybdates from their plenary precursors through ligand-directed equilibrium control in organic solvents (Fig. 1b). By reacting tetra-n-butylammonium (TBA) salts of plenary Keggin-type polyoxomolybdates [α-XMo₁₂O₄₀]³⁻ (X = P, As, V) with 4-methoxypyridine (pyOMe, C₆H₇NO) in acetonitrile, we selectively convert them into the corresponding trivacant lacunary polyoxomolybdates I_X (TBA₃[XMo₉O₃₁(pyOMe)₃]). In these products, three pyOMe ligands stabilize the structure through coordination with Mo atoms. Notably, this method enables the synthesis of lacunary vanadomolybdate I_V, a species previously unobserved under aqueous equilibrium conditions. Furthermore, the stabilized lacunary polyoxomolybdates I_X serve as precursors for assembling POM–organic hybrids II_X. Overall, this study lays the foundation for the expanded exploration and application of lacunary polyoxomolybdates in the development of functional materials, offering promising new directions in advanced POM chemistry and the design of materials for diverse applications, such as catalysis, energy conversion, and energy storage.

Results and discussion

In aqueous solutions, the equilibrium of Keggin-type POMs typically favors the formation of lacunary POMs from plenary species under high pH conditions (i.e., high hydroxide concentrations).³ Inspired by this, we explored the synthesis of lacunary polyoxomolybdates in organic solvents, beginning with the reaction of TBA₃[α-PMo₁₂O₄₀] (PMo12) with TBAOH as a base in acetonitrile at room temperature (∼25 °C). Specifically, two different molar equivalents of TBAOH (1 or 10 equivalents relative to PMo12) were investigated. When treated with 1 equivalent of TBAOH, PMo12 retained its original structure, as confirmed by both phosphorus-31 nuclear magnetic resonance (³¹P NMR) spectroscopy and electrospray ionization mass (ESI-mass) spectrometry (Fig. 2a and e). However, upon adding 10 equivalents of TBAOH, the ³¹P NMR spectrum of the reaction mixture displayed peaks at 2.52 ppm (major species) and 5.48 ppm (minor species) (Fig. 2b). Correspondingly, the ESI-mass spectrum of the mixture revealed the formation of decomposed species, identified as [P₂Mo₅O₂₃]⁶⁻ (m/z = 1155.8, [TBAH₄P₂Mo₅O₂₃]⁻) and [PMo₆O₂₂]³⁻ (m/z = 1443.8, [TBA₂PMo₆O₂₂]⁻) (Fig. 2f). These species likely correspond to the 2.52 and 5.48 ppm signals observed in the ³¹P NMR spectrum.^9a,16 Thus, the reaction between PMo12 and TBAOH did not yield the desired lacunary polyoxomolybdates but instead resulted in decomposition. To overcome this issue, we tested the addition of pyridine, intended to act as both a base and a stabilizing ligand for lacunary polyoxomolybdates. However, treating PMo12 with 250 equivalents of pyridine in acetonitrile for 1 h produced no observable structural changes in PMo12, as evidenced by its ³¹P NMR and ESI-mass spectra (Fig. 2c and g).

Fig. 2 (a–d) ³¹P NMR and (e–h) ESI-mass spectra of the reaction mixtures used for synthesizing lacunary POMs from PMo12 in acetonitrile under various conditions: (a and e) PMo12 after reacting with 1 equivalent of TBAOH for 1 h; (b and f) PMo12 after reacting with 10 equivalents of TBAOH for 1 h; (c and g) PMo12 after reacting with 250 equivalents of pyridine for 1 h; and (d and h) PMo12 after reacting with 250 equivalents of pyOMe for 15 min. (i) Crystal structure of the anionic component of I_P. (j) ESI-mass spectrum of I_P in acetonitrile. Inset: enlarged spectrum (top) and simulated pattern of [TBA₂PMo₉O₃₁]⁻ (m/z: 1875.5, bottom). Light green octahedra and the light purple tetrahedron represent [MoO₆] and [PO₄], respectively. Red, black, pink, and blue spheres denote O, C, H, and N atoms, respectively.

Consequently, to facilitate the structural transformation, 4-methoxypyridine (pyOMe, C₆H₇NO; pK_a = 14.23 in acetonitrile), a ligand with higher basicity than pyridine (pK_a = 12.53 in acetonitrile), was used.¹⁷ When PMo12 was reacted with 250 equivalents of pyOMe at room temperature (∼25 °C) for 15 min, the peak corresponding to PMo12 disappeared from the ³¹P NMR spectrum, while a new peak appeared at 0.31 ppm, indicating the structural transformation of PMo12 into a new species (Fig. 2d). The ESI-mass spectrum confirmed the formation of a trivacant lacunary structure, with a prominent set of signals centered at m/z = 1875.5, attributed to [TBA₂PMo₉O₃₁]⁻ (theoretical m/z: 1875.5) (Fig. 2h). The ESI-mass spectrum displayed another set of signals centered at m/z = 1122.7, attributed to [TBAMo₆O₁₉]⁻, confirming the formation of a byproduct, [Mo₆O₁₉]²⁻ (i.e., the Lindqvist-type cluster).

Crystallizing the reaction mixture by adding diethyl ether yielded single crystals suitable for X-ray diffraction analysis. This analysis revealed that the anionic component of the product (I_P) was a trivacant lacunary A-α-Keggin-type phosphomolybdate, [A-α-PMo₉O₃₁(pyOMe)₃]³⁻, featuring three pyOMe molecules coordinated to Mo atoms at the vacant sites (Fig. 2i and Table S1†). Notably, the ESI-mass spectrum of the I_P crystals in acetonitrile displayed a set of signals centered at m/z = 1875.5, attributed to [TBA₂PMo₉O₃₁]⁻; however, no signals corresponding to [Mo₆O₁₉]²⁻ were apparent (Fig. 2j). The ESI-mass spectrum also suggested the dissociation of pyOMe ligands from the Mo atoms during the ionization process, which occurred during the ESI-mass spectrometry analysis. Further, elemental analysis confirmed that [Mo₆O₁₉]²⁻ did not crystallize, resulting in the high-purity isolation of I_P. Comprehensive characterization using various techniques—including X-ray crystallography, ESI-mass spectrometry, thermogravimetric (TG) analysis (Fig. S1†), and elemental analysis—validated the molecular formula of I_P as TBA₃[A-α-PMo₉O₃₁(pyOMe)₃]·3(H₂O). These findings indicate that I_P was synthesized via a top-down transformation of plenary PMo12 in the presence of a base and protective pyOMe ligands, yielding [Mo₆O₁₉]²⁻ as a byproduct. This transformation is represented by the following reaction equation: [α-PMo₁₂O₄₀]³⁻ + OH⁻ + 3pyOMe → [A-α-PMo₉O₃₁(pyOMe)₃]³⁻ + 0.5[Mo₆O₁₉]²⁻ + 0.5H₂O. Density functional theory (DFT) calculations further supported the thermodynamic feasibility of this reaction, with a standard Gibbs energy change of the reaction (Δ_rG°) of −51.02 kJ mol⁻¹ (equilibrium constant, K = 8.77 × 10⁸ at 298 K).

The reaction of PMo12 with 250 equivalents of 4-cyanopyridine (pyCN, C₆H₄N₂; pK_a = 8.50 in acetonitrile), a ligand with lower basicity than pyridine (pK_a = 12.53 in acetonitrile),^17,18 in acetonitrile did not induce any structural transformation in PMo12 (Fig. S2†). However, when I_P was reacted with 250 equivalents of pyCN in acetonitrile for 1 h, followed by crystallization using diethyl ether, single crystals of PMo9-pyCN were obtained. X-ray crystallographic analysis revealed that PMo9-pyCN was structurally analogous to I_P, featuring three pyCN molecules coordinated to the Mo atoms at the vacant sites of the trivacant A-α-Keggin-type {PMo₉} structure (Fig. S3 and Table S2†). Based on the results of X-ray crystallography, ESI-mass spectrometry (Fig. S4†), TG analysis (Fig. S5†), and elemental analysis, the molecular formula for PMo9-pyCN was determined to be TBA₃[A-α-PMo₉O₃₁(pyCN)₃]. Notably, PMo9-pyCN could not be synthesized directly from PMo12via the proposed direct top-down approach using pyCN, likely owing to the ligand's low basicity and weak coordination ability. Instead, it was successfully afforded through a ligand exchange reaction with I_P, as indicated by the following equation: [A-α-PMo₉O₃₁(pyOMe)₃]³⁻ + 3pyCN → [A-α-PMo₉O₃₁(pyCN)₃]³⁻ + 3pyOMe. These results highlight the critical role of both ligand basicity and coordination ability in the ligand-directed synthesis of lacunary species derived from PMo12. To further examine this dependency, a combined approach employing pyridine and an additional base was explored. When PMo12 was reacted with 250 equivalents of pyridine and 2 equivalents of TBAOH in acetonitrile at room temperature (∼25 °C) for 1 h, the ³¹P NMR and ESI-mass spectra confirmed the conversion of PMo12 into I_P. These observations showed that the combination of a ligand and a base effectively facilitates the structural transformation of PMo12 (Fig. S6†).

To extend this ligand-directed synthesis methodology to arsenomolybdate, we examined AsMo12 (TBA₃[α-AsMo₁₂O₄₀]), a compound structurally analogous to PMo12 with heteroatom substitution. While previous studies examining the speciation profiles of arsenomolybdates in aqueous solvents have suggested the potential formation of a trivacant lacunary species,^9c,d,10b successful isolation of this species is yet to be reported. When AsMo12 was reacted with 250 equivalents of pyOMe in acetonitrile at room temperature (∼25 °C) for 1 h, the ESI-mass spectrum of the reaction solution displayed a set of signals centered at m/z = 1919.5, attributed to [TBA₂AsMo₉O₃₁]⁻, indicating the formation of a trivacant lacunary species (Fig. S7†). This ESI-mass spectrum also revealed the formation of [Mo₆O₁₉]²⁻ (m/z = 1122.6) as a byproduct. Further, crystallization of the reaction mixture using p-xylene yielded single crystals of I_As. X-ray crystallographic analysis confirmed that the anionic structure of I_As closely resembled that of I_P, with a trivacant lacunary configuration stabilized by three pyOMe ligands (Fig. 3a and Table S3†). The ESI-mass spectrum of I_As crystals in acetonitrile displayed a set of signals centered at m/z = 1919.5, attributed to [TBA₂AsMo₉O₃₁]⁻; however, no signals corresponding to [Mo₆O₁₉]²⁻ were apparent (Fig. 3b). Combined with the elemental analysis outcomes, these results demonstrate the exclusion of the byproduct ([Mo₆O₁₉]²⁻) during crystallization, ensuring the high purity of I_As. Comprehensive characterization through several techniques, including X-ray crystallography, ESI-mass spectrometry, TG analysis (Fig. S8†), and elemental analysis, confirmed the molecular formula of I_As as TBA₃[A-α-AsMo₉O₃₁(pyOMe)₃]·2(CH₃CN). The formation of I_As likely followed a reaction mechanism similar to that of I_P: [α-AsMo₁₂O₄₀]³⁻ + OH⁻ + 3pyOMe → [A-α-AsMo₉O₃₁(pyOMe)₃]³⁻ + 0.5[Mo₆O₁₉]²⁻ + 0.5H₂O. DFT calculations again confirmed the thermodynamic feasibility of this reaction, yielding a Δ_rG° value of −76.78 kJ mol⁻¹ (K = 2.87 × 10¹³ at 298 K).

Fig. 3 (a) Crystal structure of the anionic component of I_As. (b) ESI-mass spectrum of I_As in acetonitrile. Inset: enlarged spectrum (top) and simulated pattern of [TBA₂AsMo₉O₃₁]⁻ (m/z: 1919.5, bottom). Light green octahedra and the light blue tetrahedron represent [MoO₆] and [AsO₄], respectively. Red, black, pink, and blue spheres denote O, C, H, and N atoms, respectively.

Building on these findings, we sought to synthesize previously unreported lacunary structures, focusing on vanadomolybdates containing a pentavalent heteroatom (V⁵⁺), analogous to phosphomolybdates (P⁵⁺) and arsenomolybdates (As⁵⁺). Notably, lacunary vanadomolybdate species are yet to be identified in speciation studies using aqueous solvents.^9e Previous reports in this context have been limited to the synthesis of a lacunary vanadoxomolybdate capped by a triol at Mo atoms lying opposite the vacant site.¹⁹ To investigate the formation of lacunary vanadomolybdates, TBA₃[α-VMo₁₂O₄₀] (VMo12) was reacted with 250 equivalents of pyOMe in acetonitrile at room temperature (∼25 °C) for 1 h. The ESI-mass spectrum of the reaction mixture displayed a set of signals centered at m/z = 1895.5, attributed to [TBA₂VMo₉O₃₁]⁻, confirming the successful formation of a trivacant species. The spectrum also indicated the formation of [Mo₆O₁₉]²⁻ (m/z = 1122.6) as a byproduct (Fig. S9†). Crystallization using mesitylene as a precipitant yielded single crystals of I_V. X-ray crystallographic analysis revealed that I_V adopted a trivacant lacunary Keggin-type vanadomolybdate structure stabilized by three pyOMe ligands coordinated to Mo atoms at the vacant sites (Fig. 4 and Table S4†). Intriguingly, some {VMo₉} units transformed from an α-type structure to a β-type structure via an approximately 60° rotation of the [Mo₃O₁₃] unit, yielding an α/β ratio of 0.75 : 0.25, as confirmed by X-ray crystallography and ⁵¹V NMR spectroscopy (Fig. S10†). The ESI-mass spectrum of I_V crystals in acetonitrile displayed a set of signals centered at m/z = 1895.6, attributed to [TBA₂VMo₉O₃₁]⁻, with no detectable presence of [Mo₆O₁₉]²⁻. This confirmed the effective exclusion of the byproduct from crystallization (Fig. 4b). Combining the results of X-ray crystallography, ESI-mass spectrometry, TG analysis (Fig. S11†), and elemental analysis, the molecular formula of I_V was determined to be TBA₃[VMo₉O₃₁(pyOMe)₃]·3(H₂O)·0.5(CH₃CN). DFT calculations further revealed Δ_rG° values of −94.43 and −89.70 kJ mol⁻¹ for the transformation of TBA₃[α-VMo₁₂O₄₀] into the α- and β-type structures of I_V, respectively, demonstrating that the α-type structure is thermodynamically more favorable.

Fig. 4 (a) Crystal structure of the anionic component of I_V: α-type (left) and β-type (right). (b) ESI-mass spectrum of I_V in acetonitrile. Inset: enlarged spectrum (top) and simulated pattern of [TBA₂VMo₉O₃₁]⁻ (m/z: 1895.5, bottom). Light green octahedra and the yellow tetrahedron represent [MoO₆] and [VO₄], respectively. Red, black, pink, and blue spheres denote O, C, H, and N atoms, respectively.

Finally, we explored the potential of the I_X species as precursors for the synthesis of functional materials. Leveraging the reversible coordination of pyOMe ligands with the Mo atoms at the vacant sites, we investigated their reactivity with imidazole ligands, which effectively coordinate with lacunary polyoxomolybdates, as demonstrated in our recent study.^15c The reaction of I_V with 2 equivalents of 1,3,5-tris[(1H-imidazol-1-yl)methyl]benzene (L, C₁₈H₁₈N₆, Fig. 1b) in acetonitrile at 80 °C for 30 min, followed by the addition of mesitylene, afforded single crystals of II_V. X-ray crystallographic analysis revealed that the anionic structure of II_V adopted a capped monomeric configuration, comprising a {VMo₉} unit and a single L ligand coordinated to three Mo atoms at the vacant site (Fig. 5a and Table S5†). Interestingly, X-ray crystallography analysis revealed that II_V contained a mixture of α- and β-type {VMo₉} units, with an α/β ratio of 0.20 : 0.80. This indicates that some α-type {VMo₉} units from I_V (α/β ratio of 0.75 : 0.25) transformed into β-type units. The ESI-mass spectrum of II_V in acetonitrile displayed a set of signals centered at m/z = 2457.0 and 2698.2, attributed to [TBA₃HVMo₉O₃₁L]⁺ and [TBA₄VMo₉O₃₁L]⁺, respectively, confirming that the POM–organic hybrid structure was retained in the solution phase (Fig. 5b). Similarly, the reaction of I_As with ligand L afforded II_As, a structural analogue of II_V, comprising α- and β-type {AsMo₉} units with an α/β ratio of 0.08 : 0.92 (Fig. S12a and Table S5†). Further, the ESI-mass spectrum of II_As in acetonitrile presented a set of signals centered at m/z = 2480.9 and 2723.2, attributed to [TBA₃HAsMo₉O₃₁L]⁺ and [TBA₄AsMo₉O₃₁L]⁺, respectively (Fig. S12b†). These results confirm the first successful synthesis of POM–organic hybrids using trivacant lacunary vanadomolybdate and arsenomolybdate as precursors.

Fig. 5 (a) Crystal structure of the anionic component of II_V: α-type (left) and β-type (right). (b) ESI-mass spectrum of II_V in acetonitrile. Inset: enlarged spectrum (top) and simulated pattern of [TBA₃HVMo₉O₃₁L]⁺ (m/z: 2457.0, bottom) and [TBA₄VMo₉O₃₁L]⁺ (m/z: 2698.2, bottom) (L = C₁₈H₁₈N₆). Light green octahedra and the yellow tetrahedron represent [MoO₆] and [VO₄], respectively. Red, black, pink, and blue spheres denote O, C, H, and N atoms, respectively.

Conclusions

In conclusion, we successfully developed a novel ligand-directed top-down synthetic approach for producing trivacant lacunary Keggin-type polyoxomolybdates I_X (TBA₃[XMo₉O₃₁(pyOMe)₃], X = P, As, V) from their plenary counterparts TBA₃[XMo₁₂O₄₀] using 4-methoxypyridine (pyOMe) as a protecting ligand in organic solvents. Our investigations using alternative pyridine ligands highlighted the critical roles of ligand basicity and coordination ability in driving this synthesis. Notably, the proposed approach enabled the synthesis of a new trivacant lacunary vanadomolybdate, a species not previously observed in aqueous systems. Furthermore, pyOMe-protected I_X species were successfully transformed into POM–organic hybrids II_X through reactions with a multidentate imidazole-based ligand (L), demonstrating their versatility as precursors. The proposed ligand-directed synthetic strategy provides access to previously unexplored lacunary POMs and opens new pathways for the development of diverse functional materials leveraging these lacunary POMs as building blocks.

Data availability

The data supporting this manuscript is available in the ESI† of and available on request. Crystallographic data for I_P, PMo9-pyCN, I_As, I_V, II_As, and II_V have been deposited at the CCDC (deposition numbers 2408026–2408031).†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported in part by JST FOREST (JPMJFR213M), JSPS KAKENHI (24K01448, 22H04971), and the JSPS Core-to-Core program. A part of computations was performed using Research Center for Computational Science, Okazaki, Japan (Project: 23-IMS-C106, 24-IMS-C101).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Tables S1–S5 and Fig. S1–S14. CCDC 2408026–2408031. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt00252d

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